1. Field of the Invention
The present invention relates to interconnections for propagating radio frequency (RF) signals between circuit boards. More particularly, the present invention relates to a method for using surface-mount techniques to mount and solder connectors suitable for transmitting RF signals between two circuit boards.
2. Description of Related Art
Modern electronic circuits utilize components that operate in the RF domain. It is often necessary to transmit RF signals between two different circuit boards. For example, one may desire to couple the signal from a coplanar waveguide on one circuit board to a coplanar waveguide on another circuit board.
A coplanar waveguide is a type of high-frequency transmission line formed by placing a central conductor, i.e. a circuit trace, on the surface of one side of a circuit board, between two closely-spaced circuit traces held at ground potential. Additionally, a ground plane is placed on the reverse side of circuit board (opposite the central conductor), in which case the structure then becomes a coplanar waveguide with ground. A coplanar waveguide is a substantially planar analogue of a coaxial cable that has been sliced along its longitudinal axis to reveal a central conductor surrounded by a grounded conductive sheath.
In order to transmit a signal between coplanar waveguides on two different circuit boards, an interconnection must be provided between the respective central conductors and ground planes on each board. Care must be taken to keep the central conductor path well-shielded to minimize signal attenuation due to radiation of energy into the air. The design of an interconnection must also minimize reflections, another source of signal attenuation. The problems of radiation and reflection are general matters of concern in the design of RF transmission lines, of which coplanar waveguides are just one type.
Coaxial contacts, such as blind mate connector systems, have been used conventionally to provide an RF path between separate circuit boards or assemblies. However, the blind mate connector systems are bulky and expensive, and thus increase the cost per connection. They also require a manual operation to connect and disconnect them, further increasing the cost of their use.
Short metal ribbon interconnections have also been used. The metal ribbon interconnections are difficult to handle due to their small size. They also have a further disadvantage of requiring solder applications, often done manually, to complete the RF path. These disadvantages greatly increase the cost of completing each connection. Furthermore, metal ribbon interconnections cannot be easily disassembled. When one desires to disconnect two boards, the metal ribbon interconnections must be unsoldered.
A printed circuit pin and spring socket system has been used conventionally as a connection system for circuit boards for low-frequency analog, digital and power applications in electronics. A disadvantage of this system is that the pin connector is installed from beneath (i.e., opposite the component side) a circuit board and soldered in place by hand. Automated installation from the side opposite the component side requires complicated and expensive assembly equipment.
Accordingly, there is a need for an inexpensive method for installing a low-cost RF interconnection for providing a low-radiation, low-reflection RF path between multiple circuit boards or assemblies. There is also a need for an interconnection that may be installed, connected, and disconnected without manual operations.
In accordance with the teachings of this invention, a method for forming an RF interconnection between circuit boards using pin and socket connectors is provided. As described in more detail below, the present invention provides several distinct advantages over conventional methods of forming RF connections between circuit boards. According to the present invention, printed circuit pins are inserted in a circuit board from the top (component side), which is a step that is easily automated. Another advantage provided by having the pins inserted from the top side of the circuit board is that it is possible to use surface mount techniques to form solder connections between the pins and conductive traces on the circuit board. Provided the heads of the pins are thin enough to lie beneath a solder stencil, the pins may be pre-installed on the circuit board and solder applied to the pins at the same time solder is applied to other regions of the board. This eliminates the conventional manual soldering operations that are usually performed after other components have already been soldered in place. Reducing the conventional two-step, partially manual soldering routine to a single automated step provides a significant advantage of manufacturing efficiency.
The printed circuit pins used in connection with the present invention include a shaft located at the distal end of the printed circuit pin and a head at the proximal end. The shaft has a cylindrical cross-section and is dimensioned to be inserted through a conductively plated bore through a circuit board and into a socket connector. The head is wider than the plated bore through the circuit board. Thus, the head prevents the printed circuit pin from passing completely though the bore when the pin is inserted. The shaft and head are joined by a friction segment and taper region which form the intermediate portion of the printed circuit pin. The friction segment is dimensioned to fit snugly in the plated bore through the circuit board. The friction segment thus provides an interference fit through friction between the printed circuit pin and the plated bore through the PC board.
The assembly method includes a step of inserting the printed circuit pin in a bore through a circuit board. The insertion may be done by machine. The interference fit described above keeps the printed circuit pin in position while succeeding steps involving known surface mount techniques are applied to the circuit board. A solder stencil is laid over the circuit board after pins are inserted. Solder paste is applied to the solder stencil which contains several apertures through which solder paste is coated on regions of the circuit board lying beneath the apertures. Specifically, there are some apertures lying over the heads of the printed circuit pins, and the pins are consequently coated with solder paste. The solder paste is then heated and reflowed, and next cooled to complete the electrical connection between the printed circuit pin and a conductive surface surrounding the bore through the circuit board.
On another circuit board, solder paste is applied to regions surrounding bores through the circuit board. Socket connectors are then inserted through the bores. The solder paste is heated and reflowed, and then cooled to complete the electrical connection between sockets and a conductive surface surrounding the bores through the circuit board.
Once the pins and sockets are installed on their respective circuit boards, the interconnection is completed by mating the pins with the sockets. The result is a low-radiation, low-reflection RF path between the circuit board wherein the interconnection is integrally a part of the circuit boards.
A more complete understanding of the invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheet of drawings which will first be described briefly.